The field of the disclosure relates generally to fiber communication networks, and more particularly, to access networks capable of transmitting coherent optical signals.
Fiber-to-the-premise (FTTP) based access networks have been widely deployed in many regions of the world. Increasing demand for high-speed data and video services is presently driving growth in access bandwidth requirements, up to gigabits per second (Gb/s) for residential offerings and multi-Gb/s for business. Conventional FTTP network architectures utilize a passive optical network (PON), for example, a Gigabit passive optical network (GPON) within ITU-T, or an Ethernet passive optical network (EPON) within IEEE. PON is point-to-multipoint, and can be an economical alternative to point-to-point Ethernet for moderate to large populations. GPON and EPON networks have been deployed in the last 10 years, and presently realize 2.5/1.25 Gb/s data rates for downstream and 1.25 Gb/s upstream, respectively. 10-Gb/s PON (XG-PON or IEEE 10G-EPON) is being quickly deployed for high-bandwidth applications. GPON and EPON have some technical differences in terms of signal encapsulation and dynamic bandwidth allocation, but both PON types are capable of carrying data over fiber through a passive optical network all the way from an optical hub to a customer premise. Additionally, both PON types use baseband digital signaling over the fiber to carry the information.
FIG. 1 is a schematic illustration of a conventional PON system 100 for delivering PON to subscribers of a network operator. System 100 includes an optical headend (OHE) 102, a splitter 104, and a plurality of optical network units (ONU) 106 in communication with a plurality of customer premises 108, respectively. Optical hub 102 is, for example, a central office, a communications hub, and includes an optical line terminal (OLT) for converting standard signals from a service provider (not shown) to the frequency and framing used by the PON system, and for coordinating multiplexing between conversion devices on the ONUs located on or near customers premises 108.
The OLT contains a central processing unit (CPU), passive optical network cards, a gateway router (GWR) and voice gateway (VGW) uplink cards, ONUs 106 are downstream termination units for the respective customer premises 108. System 100 may be configured, for example, for 1-to-32 or 1-to-64 split ratios, over a distance of 20 kilometers, and using a fixed set of wavelengths. In a typical configuration, a PON trunk fiber 110 carries optical signals from OHE 102 to splitter 104. Splitter 104 then splits the optical signals from PON trunk fiber 110 into the different fixed wavelengths, which are then carried between splitter 104 and ONUs 106 by individual short fibers 112.
Conventional architectures like system 100, however, presently experience several drawbacks. Most OHEs, for example, have fewer PON trunk fibers available to the splitter, or node, than are required for the increasing number of subscribers. Additionally, many modern cable operators utilize a Data Over Cable Service Interface Specification (DOCSIS) infrastructure that may potentially transmit as far as 100 miles, which is considerably farther than distances supported by conventional PON technologies, which are typically limited to 20 kilometers (km). Therefore, a conventional PON extension system has been utilized to extend the transmission range of PON networks up to these increasing ranges required by a cable operator.
FIG. 2 is a schematic illustration of a conventional PON extension system 200 for deploying a PON over distances greater than 20 km. System 200 includes an OHE 202, a PON extender 204, and a plurality of ONUs 206, which may be in communication with a plurality of respective customer premises (not shown). ONUs 206 transmit and receive optical carrier signals to/from PON extender 204 by short fibers/nodes 208, and PON extender 204 connects with OHE 202 through trunk fiber 210. Short fibers/nodes 208 recover PON signal streams from PON extender 204 and transmit the recovered signals to ONUs 206 using standard PON optics. Respective nodes of short fibers/nodes 208 may also function as splitters. ONUs 206 will include 32-64 ONUs per group, and will have a symmetric architecture (e.g., ONU 206(1), 10/10G-EPON), or an asymmetric architecture (e.g., ONU 206(1)′, 10/1G-EPON).
OHE 202 includes an OLT 212, a plurality of hub transceivers 214, and an optical multiplexer 216. Hub transceivers 214 may be Wavelength-Division Multiplex Small Form Factor Pluggable transceiver (PXFP-WDM) modules. Hub transceivers 214 may also each be a combination of at least one receiver and at least one transmitter (not separately shown). Hub transceivers 214 are each configured to transmit a downstream optical signal λD to multiplexer 216, and similarly receive an upstream optical signal λU from multiplexer 216 (where multiplexer 216 also functions as a demultiplexer). Multiplexer 216 combines the plurality of downstream optical signals λD for downstream transmission over trunk fiber 210. Similarly, multiplexer 216 also splits the upstream transmission from trunk fiber 210 into the plurality of respective upstream optical signals λU.
PON extender 204 includes a demultiplexer 218, a plurality of extender transceivers 220, and a plurality of respective extender optics 222 for each extender transceiver 220. Extender transceivers 220 each include at least one digital signal processor (DSP, not shown) and are, for example, a 10G multisource agreement (MSA) transceiver module. Extender optics 222 are, for example, 10G EPON optics. Transmission between the respective hub transceivers 214 and extender transceivers 220 over trunk fiber 210 represents a PON trunk link 224. Transmission between the respective extender optics 222 and ONUs 206 over short fibers/nodes 208 represents a PON access link 226. PON extenders are sometimes referred to as “PON concentrators” due to their ability to carry multiple PONs on a single fiber between the OLT and the PON extender.
PON extension system 200 disposes OLT 212 within OHE 202, and represents a centralized architecture for utilizing Wavelength-Division Multiplex (WDM) optics, as opposed to standard PON optics with fixed wavelengths, to deploy 10G-EPON where there is a limited number fibers for the number of subscribers, and for distances over 20 km. That is, WDM technology is used to multiplex a plurality of PON streams λ onto a single fiber (i.e., trunk fiber 210). Electrical and optical interface specifications for PON extension system 200 are standardized by the Society of Cable Telecommunications Engineers (SCTE). The centralized structure of PON extension system 200 generally simplifies maintenance, reduces operational costs, and improves reliability for cable operators.
PON extension system 200, however, has several limitations with respect to scalability for the increasing per-subscriber data rates, and with respect to newer technologies used by cable operators, as well as their related services and applications. Conventional PON extender architectures not configured, for example, sufficiently to employ upcoming technologies such as next-generation PON (NG-PON, NG-PON2) based on time and wavelength division multiplexing (TWDM), which deploys at 40-Gb/s or more, or 100G-EPON, which are multi-wavelength PON systems. The conventional PON extender is unable to meet wavelength resource requirements of these newer technologies. For a PON extender to increase data transmission from 10 Gb/s to 40 Gb/s, for example, the PON extender would have to manage at least four wavelengths each in the upstream and downstream directions for every ONU, or else upgrade the 10G MSA transceivers to 25-40 Gb/s per channel with direct detection. Conventional PON extenders are not configured to manage eight or more discrete modules in parallel for each ONU, and merely upgrading a 10G MSA transceiver may significantly impair the chromatic dispersion of the signals transmitted therethrough.